Pseudomorphic high electron mobility transistors (PHEMTs) are used extensively in wireless communication systems for switching, power and low noise amplifier applications. One reason for this extensive use is that PHEMTs are generally considered to have a suitable combination of noise, power, and high frequency performance for many applications.
A typical PHEMT comprises multiple epitaxial layers grown on gallium arsenide (GaAs) substrate. Such layers may include, in sequence, a GaAs buffer layer, a super-lattice buffer layer, an optional under channel pulse doped layer, a GaAs or aluminum gallium arsenide (AlGaAs) spacer, a channel layer comprising indium gallium arsenide (InGaAs), a top spacer layer of AlGaAs or GaAs containing an inserted n-type pulse or bulk doping layer, an undoped GaAs spacer layer, and a heavily-doped GaAs cap layer for ohmic contact formation. The PHEMT also comprises a source electrode and a drain electrode formed in ohmic contact and a Schottky metal gate electrode formed on the undoped layer over the channel layer.
During typical operation of a PHEMT, respective bias voltages are applied to the gate, source, and drain electrodes to conduct electrons through the channel layer between the source and drain electrodes. The bias conditions can affect the occupation of donor and acceptor traps in the buffer, the buffer/substrate interface, and the substrate. Ionized traps act as a backside gate and influence the carrier density in the channel and thus the drain current. After a bias change, trap occupation will slowly change to the new equilibrium value, causing hysteresis in drain current.
The slow change in trap occupation upon changing bias can lead to performance defects such as drain lag, gate lag, and kink. These defects tend to decrease the linearity and the efficiency of device performance, among other things. In addition, they can prevent the effective use of power management schemes such as envelope tracking, which might otherwise be used to improve efficiency and linearity in amplifier applications. Moreover, the severity of these performance defects may vary with temperature, which can further complicate their impact on overall device performance.
In view of these and other shortcomings of conventional PHEMTs, there is a general need for PHEMTs having improved linearity, efficiency, and temperature sensitivity.